Cement used to make mortar and concrete is one of the most common building materials in the world. Despite its longtime use and scrutiny, there is still more to learn. An MIT research team led by Roland J.-M. Pellenq, Sidney Yip, and Franz-Josef Ulm is reporting a new molecular model for evaluating cement hydrate—the calcium-silicate-water mineral phase in cement—that they say is the most realistic view of the material to date (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0902180106). Previous models based on natural mineral analogs indicated that cement nanoparticles consist of long SiO2 chains interspersed with neat layers of CaO and H2O. Building on neutron scattering data reported in the literature that pinpointed cement hydrate’s Ca/Si ratio and density, the researchers created a model showing that the nanoparticles are more accurately a hybrid of crystalline and amorphous CaO-SiO2-H2O. The hybrid model contains shorter SiO2 chains with flaws that reach into the CaO layers, creating void spaces where H2O molecules gather. This minor disorder provides some atomic-level give to cement, allowing it to stretch or compress under stress, rather than snapping. “Now that we have a validated molecular model, we can better manipulate cement’s chemical structure to design concrete for desired strength, durability, and environmental qualities, such as low CO2 emissions,” Ulm says.